1. Field of the Invention
The present invention relates to an engine stall prevention apparatus for a hybrid vehicle. In particular, the present invention relates to a technique which can prevent an engine stall when the clutch is half-engaged.
2. Description of Related Art
Parallel hybrid vehicles are a type of hybrid vehicles in which electric motors are used for assisting the output of engines. In these parallel hybrid vehicles, various controls are performed, for example, as shown in Japanese Unexamined Patent Application, First Publication No. Hei 7-123509, when the vehicle accelerates, the motor assists the output of the engine, and when the vehicle decelerates, the motor generates electric power by deceleration regeneration to charge the battery. Therefore, it is possible to constantly maintain electrical energy (the remaining battery charge) in the battery and to respond to demands by the driver of the vehicle.
As described above, in a parallel hybrid vehicle, when deceleration regeneration is performed, the electric motor is driven by the rotational force of the driving wheels to function as a power generator.
However, in a parallel hybrid vehicle having a manual transmission, because the output shaft of an engine is directly connected to the output shaft of the motor, if the clutch is disengaged to disengage the driving wheels and the output shaft of the engine when the motor is performing deceleration regeneration, the torque of the driving wheels is not transmitted to the motor, and all of the torque generated by the regeneration in the motor is applied to the engine as a large load. In general, when a vehicle decelerates, since the degree of throttle opening is small and the output of the engine is limited to a small value, if deceleration regeneration is performed in such a state, there will be a problem that the engine speed may suddenly drop causing the engine to stall and the engine may be overloaded.
An object of the present invention is to provide an engine stall prevention apparatus for a hybrid vehicle, which can detect the half-engaged state of a clutch and can reliably prevent an engine stall without overloading the engine.
The first aspect of the present invention provides an engine stall prevention apparatus for a hybrid vehicle which comprises an engine (for example, engine E in the embodiment) which outputs a driving force to drive a vehicle; an electric motor (for example, motor M in the embodiment) which assists the driving force of the engine; and a power storage unit (for example, battery 3 in the embodiment) which stores electric power generated by the electric motor when the electric motor is functioning as power generator and/or stores regeneration energy when deceleration regeneration is performed by the electric motor; the engine stall prevention apparatus comprising: a clutch operation detection device (for example, steps S053 and S060A in the embodiment) which detects the operation of the clutch by a driver; a clutch state determination device (for example, steps S057, S058, S059, S060, and S061 in the embodiment) which determines the state of the clutch based on the relationship between vehicle speed and engine speed; a throttle opening degree determination device (for example, step S022 in the embodiment) which determines the degree of throttle opening of the engine; a first engine speed modification device (for example, step S010 in the embodiment) which modifies a charging/regeneration allowing lower limit engine speed value (for example, the fully closed state corresponding to the opening at the idle state in the embodiment) above which deceleration regeneration by the electric motor is allowed; and a half-engaged clutch determination maintaining device (for example, steps S008, S073, S082, and S111) which maintains a determination signal indicating that the clutch is half-engaged for a predetermined period of time, when the clutch operation detection device detects that the clutch is not disengaged, and the throttle opening degree determination device determines that the degree of throttle opening of the engine is no more than a predetermined degree, and the clutch state determination device determines that the clutch is half-engaged; wherein the first engine speed modification device elevates the charging/regeneration allowing lower limit engine speed value while the half-engaged clutch determination maintaining device maintains the determination that the clutch is half-engaged.
By constructing the apparatus as above, the engine stall prevention apparatus of the present invention increases, when a half-engaged clutch determination is maintained, the charging/regeneration allowing lower limit engine speed value, below which the charging and regeneration is not permitted. Therefore, the load applied to the engine can be reduced by stopping charging/regeneration earlier than usual, and engine stalls due to the regeneration operation in a half-engaged clutch state can be effectively prevented.
According to the second aspect of the present invention, the engine stall prevention apparatus of the present invention may further comprise a second engine speed modification device (for example, steps S077 and S078) which modifies a fuel cut lower limit engine speed value above which a fuel amount to be supplied to the engine is cut during deceleration regeneration, and the second engine speed modification device elevates the fuel cut lower limit engine speed value while the half-engaged clutch determination maintaining device outputs a determination signal indicating that the clutch is half-engaged.
By constructing the apparatus as above, when a half-engaged clutch determination is maintained, the second engine speed modification device increases the fuel cut lower limit engine speed value, and the engine speed value at which fuel supply to the engine is resumed is increased. Therefore, the load applied to the engine can be reduced by resuming the fuel supply to the engine earlier than usual, and engine stalls due to the load increase in the half-engaged clutch state can be more effectively prevented.
According to the third aspect of the present invention, the engine stall prevention apparatus may further comprise an engine start allowing device (for example, step S113 in the embodiment) which allows the electric motor to start the engine or to assist the output of the engine. The engine start allowing device drives the electric motor to start the engine or to assist the output of the engine, while the half-engaged clutch determination maintaining device outputs a determination signal indicating that the clutch is half-engaged, and the engine speed becomes lower than a predetermined engine speed value.
By constructing the apparatus as above, when the present engine speed becomes lower than the forced motor start engine speed value, the engine start allowing device drives the electric motor to start the engine. Therefore, it is possible to forcibly drive the engine when the engine is likely to stall, and engine stall can be more effectively prevented.